Download Final Report - Research

Improving CABG Surgery
Lucas Burton
Amir Durrani
Benjamin Hoagland
Santosh Tumkur
Department of Biomedical Engineering
Vanderbilt University School of Engineering
April 23, 2003
ADVISORS:
Thomas Ryan, PhD
Fellow at Ethicon Inc.
Jia Hua Xiao, PhD
Principle Engineer at Ethicon, Inc.
INSTRUCTOR:
Paul King, PhD, PE
Associate Professor of Biomedical and Mechanical Engineering
Vanderbilt University School of Engineering
TABLE OF CONTENTS
1. ABSTRACT
3
2. INTRODUCTION
2.1. OVERVIEW OF CORONARY ARTERY BYPASS SURGERY
2.2. CURRENT TREATMENTS
2.3. DESIGN GOALS
3
4
5
3. METHODOLOGY
3.1. TIMELINE
3.2. DESIGN AND PROTOTYPE
3.3. TESTING
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7
9
4. RESULTS
4.1.
4.2.
4.3.
4.4.
4.5.
ANASTOMOSIS DEVICE DESIGN
IMPROVED CABG LOCALIZED STABILIZATION DEVICE
MATERIAL SPECIFICATION
SAFETY ANALYSIS
ECONOMIC ANALYSIS
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11
12
14
14
5. CONCLUSION
15
6. RECOMMENDATIONS
15
7. REFERENCES
16
8. ACKNOWLEDGMENTS
17
APPENDIX*
A.
B.
C.
D.
E.
CALCULATIONS
DEVICE SCHEMATICS
DEVICE PICTURES
INNOVATION WORKBENCH RESULTS
DESIGNSAFE REPORT
* Click on the hyperlink to view the above appendices.
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1. ABSTRACT
Coronary artery disease is the single leading cause of death in the United States
today. Coronary Artery Bypass Grafting (CABG) utilizes a vessel to carry blood around
an obstruction in the coronary artery. The two main types of bypass surgery that are
currently being used are conventional (arrested heart) surgery through use of a heart-lung
machine and beating heart (off-pump) surgery. Our design goals concentrate on
improving the off-pump procedure. First, we improved the design of the suction foot for
the localized stabilizing device. In this novel design, we augmented the lateral
stabilization of the grafting site. Additionally, we present a device that will aid in the
attachment of the harvested vessel to the coronary artery. Second, we focused on a
device that would eliminate or reduce the number of sutures required to attach the two
vessels together using BioGlue® from Cryolife, Inc. The intravascular vessel connector
anastomosis device consists of a hollow, semi-cylindrical base with a hollow stem
attached at 45o. In this revolutionary technique, the upper shaft is inserted into the vessel
to be grafted and secured with BioGlue®. or sutures placed in a circular fashion. The
device is then inserted into an incision in the coronary artery, with the base designed to
be longer than the incision. Once the device is inserted, an outward force is applied to
stabilize the two vessels in close proximity so they can be easily sutured or joined with
BioGlue®. Our localized stabilization device consists of two suction fingers with two
chambers each. The base is mounted on a track, which expands up to 1cm via a
turnbuckle mechanism located on the posterior end of the device. The device will be
attached to the Ethicon’s FLEXSITE® stabilizer arm which allows it to be locked in
place on the surface of the heart, around the coronary artery. The turnbuckle is then
rotated away from the surgical field to spread the fingers of the device and introduce
lateral stabilization of the grafting site. The efficiency of CABG will be greatly
improved by integrating our vessel connector device, localized stabilizer device, and
BioGlue® to reduce the complexity, difficulty, and time required to perform CABG, thus
benefiting both the surgeon and the patient.
2. INTRODUCTION
2.1 OVERVIEW OF CORONARY ARTERY BYPASS GRAFTING
Seven million Americans suffer from coronary heart disease, the most common
form of heart disease. Coronary artery disease is the single leading cause of death in the
United States today (1, 2). More than 95% of all coronary artery disease is due to
arteriosclerosis, clogging of the coronary arteries by cholesterol or calcium deposits (3).
The coronary arteries deliver a constant flow of blood to the heart muscle providing it
with a necessary supply of oxygen and nutrients. When these arteries narrow or clog,
they cannot provide adequate blood flow to the heart, resulting in coronary heart disease,
which includes myocardial infarction (heart attack) and other diseases. Treatments for
coronary artery disease include medication, angioplasty, and surgery. Medications used
to treat coronary artery disease include blood thinners and drugs that decrease the amount
of work required of the heart muscle by lowering pulse rate and blood pressure.
Angioplasty is an invasive procedure that uses catheters introduced from the groin to
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work on the inside of the coronary arteries. Angioplasty procedures include the use of
stents, balloons, rotoblators, athrectomies, and lasers, to remove clots and increase blood
flow within the coronary arteries. The major type of surgery used to treat coronary artery
disease is coronary artery bypass grafting (CABG). Despite advances in the non-surgical
treatments, there are many situations in which surgery is necessary and beneficial (4).
The history of CABG began in 1876 when it was established that chest pain could
be attributed to the interruption of coronary blood supply and that heart attacks occurred
when at least one coronary artery becomes blocked (5). In 1910 Alexis Carrell presented
a paper to the American Surgical Association describing CABG, and in 1950 the first
myocardial revascularization was performed by Arthur Vineburg (6, 7). By 1968 Dr.
Rene G. Favolaro had achieved restoration of coronary blood flow in 171 patients using
CABG while working at the Cleveland Clinic (8). Today, 350,000 CAGB surgeries are
performed in the United States each year with 600,000 performed worldwide (1).
The CABG procedure utilizes a vessel to carry blood around the obstruction in the
coronary artery. The vessels used for CABG are the saphenous vein from the leg or the
internal thoracic artery from the inside of the chest. During the procedure, an incision is
made down the front of the chest through the sternum exposing the heart and aorta. This
incision is called a median sternotomy. The saphenous vein is attached at one end to the
aorta and at the other end to the coronary artery downstream of the blockage. When
using the internal thoracic artery, it is only disconnected at one end, and that end is then
attached to the coronary artery downstream of the blockage (9). The average patient can
leave the hospital approximately six days after the procedure, if there are no major postoperative problems. Patients normally take another two to three weeks to regain their
strength and normal body habits, including appetite, sleep patterns, etc. (10).
2.2 CURRENT TREATMENTS
The two main types of bypass surgery that are in use are conventional (arrested
heart) surgery and beating heart (off-pump) surgery. Typically, a patient who has
undergone conventional bypass (on-pump) surgery is discharged from the hospital
several days after surgery. According to the year 2000 Medicare claims data (11), the
average length of hospitalization for CABG surgery was approximately 8 days. During
conventional bypass surgery, patients are placed on a heart-lung machine, which
oxygenates and pumps the blood. When the patient is placed on the “bypass” heart-lung
machine, the blood is routed outside of the body through the heart-lung machine. During
this process, the patients may require transfusions to replenish blood volume, red blood
cells, or platelets. In a recent study, it was found that more than half of the patients
receiving the conventional CABG surgery required some blood products (12). Arrested
heart bypass surgery also requires considerable resources in terms of number of staff,
products and equipment, medications, and other items that will impact the total cost of
the procedure. It should also be noted that in the medical literature, surgeons refer to
“high risk patients”-patients whom they consider poor candidates for conventional bypass
surgery because they are too ill, or have preexisting medical conditions that make
exposure to the heart-lung machine too risky. These “high risk patients” may include the
elderly, and those with renal problems, diabetes, previous history of strokes or heart
attacks, etc. Patients who are unable to take part in the conventional bypass surgery may
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still undergo successful CABG surgery via a method known as beating heart bypass
surgery.
Today’s evolving surgical innovations allow surgeons to safely and effectively
suture coronary artery bypass grafts in place on the surface of the heart without hindering
its ability to beat and pump blood. These innovations have not fully replaced the
conventional CABG procedures, but they are being increasingly used, as further
technologies are developed to support them. With beating heart bypass surgery patients
may be discharged from the hospital more quickly. A recent report in the medical
literature noted a 32% shorter length of hospital stay for off-pump CABG patients (13).
The beating heart bypass surgery is performed on a beating heart through an incision
down the middle of the chest. Positioning and stabilization instruments lift and hold the
heart and then stabilize a portion of the heart’s surface where the bypass graft will be
sutured in place, all as the heart continues to efficiently beat and pump blood. Some
stabilizers use gentle suction to address the primary challenge in beating heart surgery,
making it possible to access all surfaces of the heart while reducing motion of the small
area of the surface tissue where the surgeon is sewing the bypass graft. For this
procedure, the heart-lung machine is not used. However, it should be noted that on
occasion, a surgeon might convert to the use of the heart-lung machine during the
procedure if the patient’s condition becomes unstable. One of the benefits of using the
off-pump CABG surgery method is the reduction in use of blood products that would
otherwise be required if the heart-lung machine was utilized, thus fewer patients require
transfusions when undergoing beating heart surgery. This is an important consideration
for areas where banked blood may be in short supply or where transmission of
bloodborne diseases is a concern. A study in the New England Journal of Medicine by
Newman, et al., found measurable and persistent neurocognitive decline (i.e. memory
loss, decline in thinking skills) in patients who had conventional arrested heart bypass
surgery utilizing the heart-lung machine (13). In two separate studies, postoperative
neurocognitive function test scores were significantly better in the groups of beating heart
surgery patients than in the groups who underwent conventional CABG surgery (14, 15).
Beating heart surgery potentially costs less than conventional surgery because
cardiopulmonary bypass equipment is not used, and fewer blood products are needed.
Another study found a cost reduction per patient in groups that underwent off-pump
CABG surgery. This cost reduction was directly correlated to shorter ICU use and
hospital stay. It also showed significant reduction in the need for blood products and in
postoperative complications (16). Beating heart surgery, which avoids the use of the
heart-lung machine, may make it possible for “high-risk patients” to have bypass surgery.
Lastly, a study by Arom, et al., found that off-pump CABG surgery carries a significantly
lower mortality rate in the high-risk population than conventional CABG surgery (17).
2.3 DESIGN GOALS
The main objective of this design process was to find ways to improve the current
off-pump CABG surgery procedures. Through discussions with various surgeons,
engineers, and university faculty members, we identified the two bottlenecks that hinder
control CABG efficiency. One bottleneck was associated with insufficient stabilization
at the anastomosis site. The other, more pervasive, bottleneck dealt with the difficulties
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related with the suturing of a harvested vessel to a coronary artery. The degrees of
freedom associated with effective stabilization increase dramatically if the suturing
process is either made easier for the surgeon or eliminated fully via using a vessel
connector anastomosis device. The goals of our “out of the box” approach to this design
were to provide effective vessel stabilization, simplify cardiac positioning, and minimize
hemodynamic deterioration. Small size, strength, and flexibility are the general
parameters that our group plans to work with for this next generation model. Our product
will provide stabilization and greater access to the coronary arteries, simplifying cardiac
positioning.
Currently, surgeons have been using various heart stabilizing systems made by
Guidant Inc., Medtronic Inc. and Ethicon Inc. These stabilizers are designed to address
some of the significant challenges a heart surgeon may face while performing beating
heart bypass surgery. These challenges include preserving the natural hemodynamic
function of the heart as it is held and positioned for access to hard-to-reach lateral and
posterior vessels, and then stabilizing it for precise suturing of the bypass graft. These
stabilizers also offer improved visualization and less movement of the surgical site while
still letting the heart pump blood normally.
To use the stabilizers, one must first use a heart positioner. The positioner is a
single-use, retractor-based device that features a silicone multi-appendage suction cup, an
articulating arm and a mounting clamp that is attached to the chest spreader. The suction
cup conforms to the surface of the heart at the apex. This way, the surgeon can position
and hold the beating heart to ensure access to the coronary arteries that require bypassing,
including those located in the posterior section of the heart. As soon as the heart is
positioned correctly, the surgeon can then place the foot of stabilizer parallel to the target
artery to reduce motion at the site where suturing of the bypass graft will occur.
Stabilizers come equipped with some feet that use suction, and some that rely on
mechanical stabilization alone. Stabilizer feet that use a vacuum, utilize a pull tension to
keep the grafting site stable but fall prey to losing contact with the surface of the heart if
one of the vacuum chambers should come loose. Conversely, the feet that do not make
use of the vacuum employs a push tension that helps keep the anastomosis site from
moving. Once the heart is stabilized adequately, the surgeons must then address the
challenges of creating and maintaining a bloodless field and providing continuous
perfusion to the heart muscle throughout the procedure.
We felt that for this project, we could introduce an innovative design of the
suction foot for the stabilizing device. For this novel design, we hoped to improve the
lateral stabilization of the grafting site. In order to accomplish this, we hoped to
introduce a turnbuckle mechanism, which would spread the tissue one centimeter further.
We also set out to improve the vacuum system. By implementing a new vacuum
mechanism, vacuum suction in the foot of the device would remain in contact with the
tissue despite losing one of the vacuum lines.
As a group, we decided that designing a device to aid in the attachment of the
harvested vessel to the coronary artery could be beneficial to the surgeon during bypass
surgery. We had to make certain that the device would eliminate the suturing process or
reduce the number of sutures required to attach the two vessels together. In addition, the
device needs to be made from a biocompatible material that would withstand the
pressures associated with a beating heart. The device needs to be small enough to be
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inserted into a coronary artery and must not impede blood flow between the vessels. The
edges of the device must be rounded so as not to puncture the inner wall of the
vasculature. Finally, a BioGlue® made by Cryolife, Inc. may be used along with the
device to help in the attachment with the coronary artery. If the BioGlue® attachment is
successful, the need for sutures will be eliminated.
3. METHODOLOGY
3.1 TIMELINE
Our project, improving coronary artery bypass surgery, was done between the
months of December 2002 and April 2003 as shown in Table 1. Research on the
procedure and possible ways to improve the off-pump CABG surgery began in
December. We consulted doctors and engineers and observed both on-pump and offpump surgeries. After research and consultations, some ideas for possible improvements
were put together and feedback was obtained from advisors. In mid-January, the
methods for improvement were finalized and the preliminary designs for the anastomosis
device and improved stabilizer foot were delineated. The month of February marked the
beginning of prototyping. After the initial prototypes were built, the decision of the final
material to be used in the completed designs of both of our devices was made. By
March, preliminary testing of the crude models was begun. Additionally, research and
design modifications were conducted to further tweak the design of both the anastomosis
device and improved stabilizer foot. The project was completed in mid-April leaving
time for completion of the final report and poster presentation.
Table 1: Project Timeline
December January
February
March
April
Research
Advisor Feedback
Material Decision
Design
Prototype
Testing
3.2 DESIGN AND PROTOTYPE
The design problem originally asked the team to “think out of the box” in order to
develop a more effective stabilization system. We began by acquainting ourselves with
the terrain of the chest cavity during a beating heart CABG surgery. We attended two
beating heart CABG operations and observed a standard on-pump procedure, paying
special attention to the cardio-thoracic surgeons’ hand movements and taking note of how
he interacted with the stabilization system and the beating heart system as a whole. We
consulted the Journal of Cardio-thoracic Surgery regarding beating heart surgery and
read numerous related journals in this research stage. Once we successfully grasped the
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subject matter, we consulted with surgeons and their assistants on the existing problems
that plague current stabilizers.
Brainstorming sessions were held regularly and were controlled by mandating the
introduction of at least three ideas from each group member. An engineering approach to
the problem involved logical assumptions. We wanted to stop the motion of a patch of
cardiac tissue that is about or 4.5cm x 2cm (this is the anastomosis site). There were five
ways that we could have approached this problem, or various combinations of the five
together could work to stabilize cardiac tissue locally for optimal performance.
1. Pushing on the site from either the outside or the inside of the heart to inhibit
motion (balance the forces and moments).
2. Pulling on the site from either the outside or the inside of the heart to inhibit
motion (balance the forces and moments).
3. Localized chemical methods (injection or other even less invasive means) that
would act on a small area of the heart to inhibit motion.
4. Localized electrical inhibition—something similar to shunting around a Purkinje
fiber, if that’s at all possible in the heart…
5. Apparent— motion is a perception of the mind—we would utilize a strobe light or
some other device that would apparently stabilize the anastomosis site.
Eventually, our sessions with engineers and consultations with faculty were
instrumental in identifying the two essential bottlenecks associated with beating heart
CABG surgery. The most obvious of these is the problem of insufficient stabilization.
However, a less obvious bottleneck is the lack of suturing efficiency. We reasoned that
either reducing the number of sutures or eliminating the suturing process by replacing it
with a user-friendly and efficient method would simultaneously handle both bottlenecks.
The need for increased stabilization hinges on the difficulty associated with the suturing
process; therefore, replacing suturing with an adhesive based procedure that relies on a
vessel connector would alleviate the need for more effective stabilization than current
devices provide.
Once we realized that a vessel connecting anastomosis device needed to be
designed, we held brainstorming sessions to design a novel anastomosis device. The
meetings resulted in the evolution of the device toward the final design presented in this
paper. PVC and clay models were constructed during meetings to provide visual
feedback to team members. Considerations during the design phase included: minimal
occlusion of the blood flow, biocompatibility, strength of materials, comfortable stent
angles with respect to the heart surface, damage to vessel walls, lateral suturing, and
assurances that the harvested vessel would remain fastened to the stent. Originally, our
base was a flexible oval shaped thin polyurethane material that would permit suturing.
Eventually, we decided that an extended base that was cylindrical along its longitudinal
axis would not occlude the flow and adhere to the vessel wall with greater ease. We also
discussed creating the device out of a bio-absorbable material, but this option was
dismissed due to its tendency toward forming emboli.
The reason that
Polyetheretherketone™ (PEEK™) was chosen as the primary polymer for the vessel
connector can be attributed to PEEK™’s ability to withstand a significant amount of
tensile and compressive stresses; furthermore, PEEK™ is biologically inert. Ideally, the
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second bottleneck would also be handled with the introduction of an efficient adhesive
that would replace the suturing via concurrent use with the vessel connector; Cryolife’s
BioGlue® assumes this role in our ideal effective beating heart CABG system.
After handling the primary bottleneck, we chose to improve the stabilization by
refining the current turnbuckle based foot model. This included identifying the main
problems in the current foot design and then replacing them with improvements.
Therefore, we introduced a parallel port vacuum system that would not falter during
surgery as the current series port vacuum system does. Lateral tautness was emphasized
to increase local stabilization. An easy access turnbuckle system on the posterior end of
the stabilizer foot would provide surgeons the means to increase lateral tension by
spreading the cardiac tissue beneath the suction. Another advantage to this novel design
was an introduction of a degree of freedom with respect to the width of the view window;
the turnbuckle mechanism provided a method of adjustment for the view window width.
3.3 TESTING
Foot Design Pressure Approximation
We began on the assumption that the anastomosis site would be 3.5cm by 2cm
(symbolically l  h ). Moreover, the heart is simplified as a fluid filled sphere, of a
diameter d, possessing a wall thickness t. The pressure of the fluid is FL and distributes
over the inner surface of the beating heart. The muscular walls of the simplified heart
exert a force to counter the beating motion of the heart; this force will be denoted as Fm,
which is correctly associated with a resistive material force that occurs in similar systems.
The pressure associated with the resistive muscle force (σh) is the Hoop stress in this
problem. Solving for the Hoop stress:
d 2
FL  P 
4
Fm  dt    h
 
 d 2 
d
 4 
Pd
h 
4t
Introduction of the stabilizer into the environment permits us to define a new
force, the suction force that we desire (Fs) for sufficient stabilization. A pressure gradient
exists across the heart wall during relaxation and contraction; where the pressures
associated with the former state are collectively P2 and the pressures associated the latter
state are collectively P1. The force of the blood at the anastomosis site will be labeled Fp.
A force balance is set up to determine an adequate Fs, where the inherent resistive
muscle force is set equal to the sum of Fp and Fs:
 h  dt  P  
Fm  2l  h t h
F p  l  h P
Fs  Ps l  h
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2l  ht h  Pl  h  Ps l  h
Ps ~ 2P or approximately 240 mmHg
therefore, this heart contact member is configured to facilitate the use of negative
pressure to engage the surface of the heart.
For images of simplified heart model, see appendix.
Vessel Connector Anastomosis Device Material Calculations
The surface area of the device is 196.1mm2 (1.961e-4m2). We shall approximate
the device as a flat plane of the same surface area. Based on previous experimentation,
the maximum pressure inherent in the beating heart system that will affect the fidelity of
the device is 560mmHg or 7.46e4 N/m2. Using this upper pressure limit will permit us to
compensate via both tensile and compressive strengths.
The base will apply an axial force against the wall of the vessel, at an angle
vertical to the longitudinal axis of the protruding, beveled shaft. Moreover, the base is
designed to apply a radial force at a horizontal angle transverse to the shaft’s longitudinal
axis, against the wall of the target vessel, to secure the device to the target vessel.
4. RESULTS
4.1 ANASTOMOSIS DEVICE DESIGN
The vessel connector design, as seen in Figure 1, serves as an anastomotic stent
for connecting a graft vessel to a target vessel, particularly for use in coronary artery
bypass grafting. The design comprises a small
vessel anastomotic stent for use on a target
vessel, which has a small diameter roughly
equivalent to that of the coronary artery. The
base of the device consists of a semicircular
cylindrical piece with a diameter of 2mm and
length of 10mm. The wall thickness is
0.1mm. Along the 10mm side of the base, the
edges are rounded at a radius of 1.5707mm. A
stem, 2mm in diameter, is attached to the base
at an angle of 45o relative to the base. A hole
passes from the cylindrical stem to the base. Figure 1. Anastomosis Device
At the attachment point between the base and
stem, the edges are rounded with the 135o attachment sloped to the base at an inverse
radius of 3mm and the 45o attachment point sloped to the base at an inverse radius of
0.6mm. This allows for a smooth attachment of the harvested vessel to the coronary
artery.
The stem is a hollow cylinder with a 2mm diameter and 0.1mm wall thickness.
The stem’s length is approximately 10mm. On the stem are two rings with semicircular
cross-sections 1mm in diameter. The rings are separated by approximately 2mm,
midway down the stem and slope to the surface at an inverse radius of 1mm. The area
between the rings provides a site for securing the harvested vessel to the device with
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lateral sutures. At the top of stem is a ring, also with a cross-section of 1mm that slopes
to the surface of the stem at an inverse radius of 3mm. The purpose of this ring is to
create a seal to prevent blood flow from escaping outside the device to the thoracic
cavity. All of the edges of the device are rounded so as not to puncture the internal wall
of the vasculature.
The shaft will be inserted into the vessel that is to be grafted onto the coronary
artery and secured in place using BioGlue® or suture. The suture will be placed in a
circular fashion around the vessel between the raised beveled rings. This will prevent the
vessel from detaching from the device. Once this vessel is securely attached, the semicircular base will be inserted into the coronary artery. The base is designed to be longer
than the length of the incision so that it will not slip out. Once the device is inserted into
the coronary artery an outward force, perpendicular to the artery surface, will be applied
to the device so that the base will be brought into contact with the upper inside surface of
the coronary artery. The two vessels are now stabilized in close contact with one another
and can be easily sutured together or attached with BioGlue®.
4.2 IMPROVED CABG LOCALIZED STABILIZATION DEVICE
The improved localized stabilization device, as seen in Figure 2, includes a
modification to the suction foot stabilizer
produced by Ethicon, Inc. Its main advantage
over existing models is that it provides
improved lateral stabilization to the site of
anastomosis.
The chief addition is a
turnbuckle to the posterior end of the suction
foot to allow the foot to be spread up to
approximately one centimeter beyond the
initial location. The turnbuckle is spread
using a finger or thumb and can be easily
rotated with gloved surgeon hands.
Moreover, due to the threaded nature of the Figure 2. Localized Stabilization Device
turnbuckle, once in place, the feet will remain
in their location until the turnbuckle is reversed. The turnbuckle will allow the foot to
expand along a track an additional centimeter beyond its initial configuration. The
turnbuckle has a silicone covering to allow easy movement by the gloved, surgeon’s
hand.
The localized stabilizer foot device will attach to Ethicon’s FLEXSITE® arm for
use in local stabilization of the surgical site during coronary artery bypass surgery. When
attached to the FLEXSITE® arm, the foot can be rotated and swiveled nearly 360 o in two
planes. Once placed in the desired position, the grip of the stabilizer arm is released,
which releases a cable that tenses both the arm and the foot in place. The entire
FLEXSITE® Stabilizer becomes rigid including the position of the foot.
The current design of Ethicon’s suction foot incorporates a malleable base with
suction areas on both fingers of the foot. Once placed on the heart, the foot cannot be
manipulated or adjusted. The suction pulls the tissue away from the heart’s surface to
stabilize the site. In reference to the suction system, Ethicon’s current suction foot
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utilizes two suction cells on both of the suction fingers for a total of four suction cells. A
common 5mm silicone surgical tube feeds the foot from a vacuum source collection
reservoir. This vacuum tube splits into two 2mm tubes that feed each of the fingers of the
foot. A small, 0.5mm tube made into the inside of each finger supplies each of the
suction cells of the foot.
An additional improvement is the addition of an improved vacuum system. This
improvement will eliminate the current system’s problem where the entire device
becomes dislodged when one cell looses tissue contact. The reason for this problem is
that the vacuum lines feeding each of the cells have a high resistance when attached to
the tissue. When one cell becomes dislodged, its resistance drops severely which causes
the vacuum to pull most of its suction through its line. This greatly reduces the suction
created in the other cells, and they dislodge. Our system will have each of the cells fed
by an independent vacuum line from the vacuum source collection reservoir. This “sink”
will allow each of the cells to have a constant suction even if one of the other cells
dislodges from the tissue. An analogy can be drawn between the old system and a circuit
with a voltage source and four resistors, of equally high resistance connected back to the
voltage source. When one of the resistors is changed to low resistance, most of the
current flows through that resistor. Our design can be compared to a voltage source
connected to four resistors in parallel and then grounded. The current flowing through
each of the resistors is constant regardless of the resistance of the other resistors. Our
design will assure a stable surgical field even if one of the suction cells becomes
dislodged.
Our design maintains the low-profile nature of the current suction system and
continues to provide the high visibility boasted by the Ethicon’s current foot device.
Moreover, since our foot fits on to Ethicon’s current FLEXSITE stabilizer arm, surgical
setup is easy and fast. Our design can be easily integrated into Ethicon’s system and has
the potential to join Ethicon’s line of foot designs
4.3 MATERIALS
Anastomosis Device Material
The lower limit of Polyetheretherketone’s™ (PEEK™) tensile strength rests at
70MPa or 52.5e4mmHg. Indicating that PEEK™ is not reinforced will suffice for the
vessel connector, but the degrees of freedom increase dramatically with carbon fiber
reinforcement. With 30% carbon reinforcement the connector would experience
optimum wear resistance and load carrying capability (90e4mmHg). The following
calculation indicates that our device, composed of PEEK™, demonstrates a tensile
strength well above the 560mmHg pressure constraint:
52.5e4 mmHg/ 560 mmHg = 937.5
therefore, our design should be able to withstand nearly a thousand times more tensile
strength than the pressures the CABG environment could generate.
PEEK™ polymer is extensively used in medical applications. It combines
strength, purity, chemical resistance and ease of processing with superb sterilization
resistance and a lack of interaction with biological systems. Moreover, PEEK™ is
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widely regarded as the only material, which is easily processed and has the ability to be
steam-sterilized repeatedly without a significant reduction in physical properties.
Based on the cost of PEEK™ (approximately $90 US/kg) at a density of
1.26g/cm3, the material costs associated with production of 1000 devices (at 20.22mm3
volume) amount to approximately $2.80.
Table 2: Polyetheretherketone™ Properties
Tensile
(73 °F)
Strength
Flexural
(73 °F)
Strength
Pa
Pa
Compressive
Strength
(10% Deflection)
Pa
9.99e7
1.70e8
1.17e8
Shear Strength (73 °F)
Limiting PV
Pa
Pa/FPM
5.30e7
1.38e8
Cryolife’s BioGlue® Adhesive
The glue is of high strength, stronger than sutures, and forms a non-toxic bond in
30 seconds or less. Wet conditions do not impede the formation of the bond. The
compositions provide strong and rapid bonding to a range of substances of both natural
and synthetic origin. BioGlue® provides rapid bonding to vascular and cardiac tissues,
but strong bonds are also formed to leather, rubber, Dacron®, Teflon®, as well as metals.
This enables the adhesive to be used for the attachments of surgical grafts and devices, as
well as for wound closure, trauma repair, and hemostasis. This particular combination of
properties is ideally suitable to the CABG environment (18).
The adhesive compositions are the results of cross-linking on a surface or surfaces
to be bonded of a mixture comprised of - Part A.- a water soluble proteinaceous material
of about 27-53% by weight of the mixture; Part B.- di- or polyaldehydes present in a
weight ratio of one part by weight to every 20-60 parts of protein present by weight in the
mixture and water, optionally containing non essential ingredients to make up the balance
of the composition. The final cross-linked bonding compositions are water insoluble,
rubbery or leathery proteinaceous solids substantially free of aldehydes, and adherent to
the substrate to be bonded with a tear strength of at least 75g/cm2. Bonding is
accomplished by combining the two-part system (the parts being referred herein as Part A
and B, respectively), and allowing the mixture to react on the surface or surfaces that are
being joined. Rapid bond formation follows, generally requiring less than one minute to
complete. The resulting adhesion is strong, generally providing bonds with tear strengths
of 400-600g/cm2. Tear strengths of 1300g/cm2 have been recorded (18).
Localized Stabilization Device Materials
Based on previous experimentation, the maximum pressure inherent in the beating
heart system that will affect the fidelity of the device is 560mmHg or 7.46e4N/m 2.
Metals, which are biocompatible and meet the strength requirements for this application
include: titanium (tensile strength of 2.76e8N/m2 at room temperature), tantalum (tensile
strength range from 2.41e8N/m2 to 4.83e8N/m2 at room temperature), and surgical
stainless steel (4.48e8N/m2). Superelastic materials such as nickel titanium alloys are
reasonable materials to use in the foot as well.
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4.4 SAFETY ANALYSIS
Throughout the entire design process, all possible safety problems that might arise
were considered. To aid in this process, we used the Designsafe™ program to help us
limit or solve all the possible problems. We determined that there might be some safety
issues regarding the stabilizer foot. One of the problems that could arise is the possibility
that the stabilizer foot could cause a tear in the tissue of the heart when it spreads. To
overcome this, we designed the foot to only spread a distance (~1cm) that would not
overexert the tissue to a point where it would tear.
The other main safety problem with the stabilizer foot is the vacuum system. The
older version of the device had four chambers in which the two pairs of chambers were
essentially connected in series with one vacuum tube each. Therefore, if one of the
vacuum tubes malfunctioned, then the others would loose suction and contact with the
heart’s surface would be lost affecting the heart hemodynamics and suture insertion
during the surgical procedure. In order to bypass this problem, we designed a stabilizer
vacuum system in a parallel fashion in which each of the four chambers is fed by a
separate vacuum tube that stems from the main vacuum source.
The anastomosis device presented us with more challenging safety concerns. The
device was to be used invasively and thus the patient’s health was our first priority. We
make certain our final design did not possess any sharp edges, which could puncture the
inner wall of the vasculature.
Another issue with the anastomosis device was biocompatibility. In order to use
this device, it had to be compatible with the patient’s body. After researching possible
materials, we decided that the use of PEEK™ would be our best option, but we have not
excluded the possibility of using titanium, stainless steel or tantalum. PEEK™ as well as
the other materials would still require extensive testing before being used in surgical
practices. PEEK™ was chosen for the following properties: high tensile and flexural
strength, high fatigue limit, high impact strength, low flammability, and its inability to
interact with a magnetic field. The use of PEEK™ also addresses the potential problem
of the device breaking and causing an embolus. Due to PEEK’s™ superior tensile
strength and high fatigue limit, the forces incurred in the heart would not be sufficient to
break the vasoconnector device.
The task of proper placement of the anastomosis device in the heart would fall on
the surgeon. It is his or her responsibility to have the proper training with the device to
use it correctly. We designed this device on the assumption that the surgeons’ who use it
have learned the proper protocol on installing the device. We feel that the design of the
device is very straightforward and user friendly and should not pose any significant
problems to the user.
4.5 ECONOMIC ANALYSIS
If we use PEEK™ as the polymer of choice for our vessel connector device, we
can determine a fairly accurate cost of making our device. We found that PEEK™ has a
cost range from $90-$110 per kg and a density of 1.26g/cm3. Using these figures and the
fact that our device has a volume of 20.22mm3, the material cost of making 1000 of the
vasoconnectors is approximately between $2.30 and $2.80. It is very inexpensive to
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make a great quantity, due to its very small size. It must be noted that there could be
significant costs in the development and testing of the device to ensure proper function.
As of now, we only have a scaled up (10x) prototype of our anastomosis device
built. In some places, this prototype would cost up to $360. Working through Vanderbilt
University and the NCIIA, we had our prototype built by the University of Pittsburgh.
When this device nears its final stages of development, more resources would be needed
to conduct rigorous analyses of the structural integrity and performance characteristics of
the device. For testing purposes we would need to get FDA approval to conduct trials
with animal specimens before the device is to be utilized in humans.
With 150,000 off-pump CABG surgeries performed worldwide every year, there
is a significant need for this device. The device is inexpensive to produce and easy to
use. It may eliminate the need for sutures and make the surgical procedure less difficult.
Using this device might also make the CABG procedure less expensive.
5. CONCLUSION
We have successfully designed an anastomosis device that will reduce or
eliminate the suturing required to perform the CABG procedure by providing a simple,
quick and effective method to bring two vessels in close proximity to one another.
Elimination of suturing during the procedure can be achieved through use of BioGlue®
in conjunction with our vessel connector device. We have also developed an improved
foot stabilizer device enhances lateral tension and provides a superior suction system to
provide greater local stabilization at the anastomosis site. The efficiency of CABG will
be greatly improved by integrating our vessel connector device, our foot stabilizer device,
and BioGlue® to reduce the complexity, difficulty, and time required to perform CABG,
thus benefiting both surgeon and patient.
6. RECOMMENDATIONS
Future Testing (Materials and Methods)
The anastomosis device should eliminate or reduce the amount of suturing during
the anastomosis procedure via cooperation with the novel stabilizer foot design and
Cryolife’s BioGlue®. BioGlue® is an adhesive composition comprised of cross-linked
proteinaceous material.
In Vitro
Either bovine or swine hearts must be procured and the coronary arteries should
be rinsed with saline-heparin solution. Segments of human saphenous veins will be used
as the conduit vessels. Anastomoses will be created. In brief, the saphenous vein and the
left anterior descending coronary (LAD) are to be positioned in close proximity. The
BioGlue® adhesive should be applied via its standard applicator gun and tip. The
saphenous vein will be slipped over the beveled shaft of the vessel connector/anastomosis
device and more BioGlue® will be applied to the proximal region of the shaft where the
saphenous vein meets the artery surface. After a sufficient period of time (30 seconds
ensure 65% bonding strength) has passed, an incision of reasonable size is to be created
in the coronary recipient, the vessel connector base will be inserted into the artery and
15
pulled outwards in order to ensure contact between the device base and the artery wall.
The proximal end of the vein graft, as near to the device as possible, should be connected
to a pressure-transducing box. Once flow through the vein graft is established via saline
injection, the vein graft and the coronary artery will be perfused with a large syringe
connected in a Y fashion to the proximal vein graft and the pressure transducer, and
saline solution will be forcefully injected to simulate a pressure of at least 300mmHg
(19).
In Vivo
After about 50 anastomoses, in vitro, are conducted, a long-term animal model
needs to be established. Yearling goats or mongrel dogs are possible experimental
animals. The left anterior descending coronary artery should be dissected and proximal
and distal sites of the future arteriotomy will be occluded via elastic snares. Direct
anastomosis will be performed between the LITA (left internal thoracic artery) and a
5mm coronary arteriotomy on the beating heart; the novel localized stabilization foot
design detailed above will provide localized stabilization that will promote a comfortable
working environment for the surgeons. The ITA (internal thoracic artery graft) will be
fitted over the vessel connector shaft in the same manner as was detailed in the in vitro
section. Five cc BioGlue® will applied and allowed to dry for 2 min. The distal elastic
snare should be partially lifted to allow slow progressive retrograde filling of the vessels
with some extra vascular oozing. The distal elastic snare must then be removed and the
ITA-graft opened. Definitive proximal occlusion of the LAD upstream will exclude
competitive flow from the native coronary (20).
Additionally, we would have to perform extensive stress and material testing of
the anastomosis device and foot and investigate other possible materials for the
anastomosis device. We could also explore other possible vessel connector designs or
improvements and obtain further feedback from doctors.
7. REFERENCES
1. “Coronary Heart Disease.” 2002 Heart and Stroke Statistical Update, Dallas,
Texas. American Heart Association, 2001. http://www.americanheart.org,
2. “Facts About Coronary Heart Disease.” National Institutes of Health.
http://nhlbi.nih.gov, (27 July 2001).
3. “The Heart and Coronary Artery Disease.” Hall-Garcia Cardiology Associates.
http://www.hgcardio.com, (27 July 2001).
4. http://www.heartpoint.com/treatcoronaryartdis.html
5. Westaby S, Landmarks in Cardiac Surgery. Isis Medical Media, Oxford, 1997.
187
6. Ibid., 230
7. Borst, H & Mohr, F, History of Coronary Artery Surgery – A Brief Review; J
Thorac Cardiov Surg 2001; 49: 195-198
8. Ibid, 196-197
9. http://www.sts.org/doc/3705
10. http://www.enter.net/~fsadr/cabg.htm
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11. Hanvik B., Public Relations, Medtronic, Inc. Minneapolis, Minnesota.
12. Puskas JD, et al. Clinical outcomes, angiographic patency, and resource
utilization in 200 consecutive off-pump coronary bypass patients. Annals of
Thoracic Surgery 2000;71:1477-1484.
13. Newman, et al. Longitudinal assessment of neurocognitive function after
coronary-artery bypass surgery. New England Journal of Medicine
2001;344(6):295-402.
14. Diegler A, et al. Neuromonitoring and neurocognitive outcome in off-pump
versus conventional coronary bypass operation. Annals of Thoracic Surgery
2000;69:1162-1166.
15. BaskerRhao B., et al. Evidence for improved cerebral function after minimally
invasive bypass surgery. Journal of Cardiovascular Surgery 1998;13:27-31.
16. Boyd W., et al. Off-pump surgery decreases postoperative complications and
resource utilization in the elderly. Annals of Thoracic Surgery 1999;68:14901494.
17. Arom K., et al. Safety and efficacy of off-pump coronary artery bypass grafting.
Annals of Thoracic Surgery 2000;69:704-710
18. U.S. PATENT DOCUMENT 5,385, 606 A 1/1995 Kowanko
19. Gundry, S. R., Black, K. and Izutanii, H. Sutureless artery bypass with BioGlue®
anastomoses: preliminary in vivo and in vitro results. Journal of Thoracic and
Cardiovascular Surgery, 2000, 120, 473–477.
20. Van Nooten, Y. Van Belleghem, L. Foubert, K. François, F. Caes, H. Van
Overbeke and Y. Taeymans, An experimental model of coronary anastomosis
without suturing. Cardiovascular Surgery, 2003, 11, 80-84.
9. ACKNOWLEDGMENTS
We would like to thank the following people for all their help in our design project.
Without their help, most of this would not have been possible.
Thomas Ryan, PhD
Paul King, PhD, PE
Jia Hua Xiao, PhD
Walter Merrill, MD
James Greelish, MD
V. Anilkumar, PhD
E. Duco Jansen, PhD
Carol Rubin, PhD
Robert J. Bayuzick, PhD
Bruce Hoagland, MBA
Randall Ryan
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